"Upon report of these incidents reaching the office, double watches are assigned to shadow both cellar and cabin occupants. It becomes evident that the cabin tenant is simply spying upon conduct of the others. Fearing that any decisive attempt to learn his 'lay' may work unnecessary complications, he has not been molested.
"This same Thames programme and tableau were enacted each of the two succeeding nights. On last afternoon, shortly before dusk, both men came up the stairs. They walk along together for a while, when the elder stops at a stall where loaves of bread are exhibited. One of our agency men is just ahead, lounging along lazily, but intently listening. The elder, who slightly limps, softly says:
"'Get back early, Paul!' then glances nervously ahead. In subdued whisper comes the reply, 'Yes, father.'
"That evening former performance is repeated. This important clew was reported at headquarters shortly before your call.
"It cannot be otherwise than that Pierre and Paul Lanier are in London, occupying the basement room down those old stairs. Paul makes these night trips up the Thames to scene of his crimes. His conduct stamps him as the murderer of Alice Webster and Oswald Langdon."
Sir Donald holds his peace while shrewd guesses are made as to causes of such suggestive actions.
Still referring to his memoranda, the chief continues:
"Paul is partially deranged. The bodies pitched over the steep bank, and he imagines will escape. Knowing that Alice Webster had been rescued from the lake, he fears she may rise from Thames depths. Pausing at shrubbery along the shore and scrutinizing of projecting branches is through knowledge of how she was saved from that lake immersion. Perhaps Paul is sane on all subjects except the murders. Even as to these he may manifest much craft. Such crazed freaks sooner or later will lead to sure exposure. Pierre knows his son's disordered mental state. It is only necessary that both be well watched. Paul's irresponsible craze will do the rest. The 'lay' of this spy can only be surmised. Perhaps these villains are suspected of other crimes. It is improbable that any self-constituted detective is on their trails. However, this sleuth will be persistently shadowed. It is possible that thereby some important 'find' may occur. By such course our bureau will hedge against all interference."